In computer graphics, the term “bitmap” means “map of bits” or a spatially mapped array of bits, and refers to a type of file format used to store digital images. Raster images in general may be referred to as bitmaps whether synthetic or photographic, in files or in memory. In some contexts, the term bitmap implies one bit per pixel, while the term “pixmap” is used for images with multiple bits per pixel. Halftone prints can be produced from bi-level images that indicate the presence or absence of ink at a particular location within the image. Variation of halftone dot sizes controls tonal values within a clustered-dot halftone print. A halftone image file can be viewed as a bitmap. Each bit is used to indicate whether a pixel is on or off.
Color halftone printing devices make use of different channels, e.g., Cyan (C), Magenta (M), Yellow (Y), and Black (K), to define colors. Thus, color halftone prints may include a plurality of monochromatic bitmaps. Each monochromatic bitmap defines the presence or absence of dots for a particular channel. Color management techniques may be applied to halftone prints by modification of continuous-tone (contone) image data prior to halftoning. Color management via modification of contone image data requires that pages be rendered twice, once for a proof and once for printing plates. However, rendering pages twice can result in different dot structures on the proof and the plates.
When printing, for example, using a color inkjet printer, the color is applied at once to the paper. Each color generally ends up where it is expected. When printing, in commercial printing presses, a document consisting of more than one color of ink requires that the single page pass through the printing press multiple times as each color is applied to the paper. Sometimes the paper or the plates applying the ink shift. It might be a tiny shift but sufficient to be noticeable. For example, a white gap may appear between a cyan letter that is supposed to be touching a magenta box. When this happens, the color is said to be out-of-register. Color-to-color misregistration refers to misregistration between color separations in a printed image. Such misregistration in printed halftoned images can result in image defects, a primary one being white gaps between color edges.
Methods for correcting for misregistration are known. The general approach is to expand one of the abutting regions' separations to fill the gap or misregistration border region with a color determined to minimize the visual effect when printed. Borders or edges expanded from a region of one color to another in this manner are said to be “spread”. A border which has been expanded is referred to as a “trap”, and the zone within which color is added is called the “trap zone”. Misregistration of colors in a printing device can, in some instances, be compensated for by color “trapping”. Trapping is the process of compensating for misregistration by printing small areas of overlapping color where different colored objects meet. Trapping makes these gaps less noticeable, even invisible. Trapping is often accomplished with features built into software algorithms devoted to trapping methods. Many commercial printing devices perform a trapping method.
One trapping method is to simply print using all one color or never allow your colors to touch or get too close. This may not be practical for many print jobs. Another approach is to intentionally print one layer of ink on top of another. This is called overprinting (also known as surprinting). Overprinting is used to help avoid the need for color trapping by intentionally avoiding gaps between touching colors. Depending on the colors printed, varying degrees of noticeable changes may arise in color where the two objects overprint. In some instances, overprinting may be desirable and the results better than trapping. Small amounts of overprinting are often less noticeable than the white space that might occur with untrapped knockouts and may be easier to achieve than trying to set a perfect trap. Overprinting is usually not suitable for non-black colors. For example, when dealing with a cyan and a magenta border, using overprint will change one of the colors to blue due to overlapping of cyan and magenta.
Many image documents are RIP'ed to bi-level data out of considerations for memory and performance. RIP'ing means to be Raster Image Processed. Raster image processing turns vector digital information, such as a PostScript file, into a high-resolution raster image (RIP'ed bi-level bitmap). A Raster Image Processor (RIP) is a relatively common component in document reproduction systems.
Accordingly, what is needed in this art is a color trapping algorithm based upon a halftoned high-resolution bi-level bitmap to effectively deal with color misregistration in complex digital document reproduction devices and to reduce other engine artifacts, such as trailing edge detection, where color intensity falls off sharply at the trailing edge.